Users of stoves and furnaces which burn solid fossil fuels as wood or coal (collectively referred to hereinafter as wood burning furnaces) have long had to contend with their deleterious safety aspects (such as creosote fires in the discharge flue) and their need for constant attention to properly control combustion and insure that the correct amount of fuel and oxygen is present. In recent times as the cost of electricity and natural gas has markedly increased, more people have begun using wood burning furnaces as economical supplemental or principal heating in environments as non-farm homes. In these environments the furnaces are commonly located remote from the usual places at which users spend the bulk of their time, substantially increasing the likelihood that combustion may cease without the immediate awareness of the user, possibly precipitating other damage to the structure or its contents before suitable temperatures may be restored. Moreover, no matter where its installed location, a furnace heretofore had to be physically inspected for safety reasons and to insure continual heating operations. Significant amounts of energy would almost inevitably be released with each inspection, often involving physically opening an access to the furnace combustion chamber.
To combat one of the most dangerous of the safety hazards presented by wood burning furnaces, the creosote flue fire, a type of monitoring device was developed which simply provides an alarm when the flue temperature exceeds some preselected flash point. However, since, depending upon creosote buildup, fires may occur in the flue somewhat irrespective of furnace combustion conditions (e.g., temperature, flame, combustion rate, fuel/air ratio, etc.), such devices cannot furnish the user with any information regarding the operating conditions of combustion within the furnace.